Think back to the last time you walked into a room, flipped the light switch, and were greeted by a bright flash from the bulb and then darkness. Your bulb has burned out! Nothing lasts forever and light bulbs are relatively affordable, but why do they give off that big flash before they die? The light bulb swan song is one of those things that you maybe only witness a few times a year, but everyone has experienced it, likely with little thought. But why do light bulbs go out with such a (albeit silent) bang?

Note: We are only talking about incandescent light bulbs here. Your fancy LED or Halogen light bulbs don’t feature such excitement.

To understand the flash, we need to understand how incandescent light bulbs work. Light bulbs have a thin filament, through which electricity flows. Electrical currents don’t flow through wires at a 100% efficiency, meaning there are byproducts. The more resistance, the more waste there is. In the case of a toaster, the waste is almost only heat. The coils of a toaster are designed specifically to give off an appropriate amount of heat to toast your bread or bagel. They give off light too (that’s why they glow orange), but heat is the goal. Those coils are far less conductive than, say, the wires that carry electricity through your home behind the walls (thankfully).

Incandescent light bulbs work in much the same way. The coil (or filament) in a light bulb is designed to get hot as well. But with light bulbs, we want them to get white hot, so that they give off light. The most common filament is tungsten, because it has an extremely high melting point. This means it can reach very high temperatures and stay as a coil within the bulb. The glass bulb protects that white hot filament. If the filament was surrounded by air, it would burn, so light bulbs are filled with an inert gas.

The amount of light giving off by the wire is proportional to the amount of resistance it has which, in turn, is proportional to the thickness of the wire. The more thin a wire is, the more resistance the electricity has to pass through. More resistance means more heat and light. As a light bulb ages, the tungsten wire oxidizes and gets thinner and thinner, resulting in a gradual increase in the amount of light and heat given off. While this starts as a slow process, it gets faster over time.

 

Eventually, the filament gets so thin that it breaks. But why does this happen right when we turn the light on? To really understand that, we have to understand a bit about the thermodynamics of tungsten. As it heats up, tungsten gains resistance, which in turns allows it to better handle the electrical current flowing through it. A nice and healthy filament can handle the rapid influx of electricity while its resistance is being ramped up.

This all happens in the moments after you turn it on. However, once the filament is worn down and uneven (again, this happens as the tungsten oxidizes over time), it isn’t able to handle all the electricity! It can’t dissipate the heat fast enough and the thermal stress causes it to expand! But it’s already compromised and uneven, so that shock (pun intended) has a catastrophic failure. It is breaking before it warms up. In that moment that it fails, the filament has a very high resistance and gives off a huge amount of light and heat. That’s where the flash comes from. It’s an arc between the newly broken pieces of tungsten.

Now that we’ve gotten to the bottom of that, what other mysteries are left in the world?